An ignition control system for a multi-cylinder reciprocating engine controls the ignition function of the engine and more particularly determine both the dwell and spark times of the ignition system. The dwell time determines how much energy is to be stored in a coil and subsequently discharged to the spark plug. The spark time determines when the energy will be discharged from the coil into the spark plug to ignite the fuel/air mixture within the cylinder.
Early automobile ignition control systems utilized a mechanical system which included a single spark distribution system (e.g., a magneto and associated mechanical distribution hardware) and a single spark plug per cylinder. Historically, these systems were plagued by reliability problems with the spark distribution system. In addition, these systems are susceptible to mechanical failures which led to reduced engine performance based on excessive normal wear and tear and in the most severe case destruction of the engine. These mechanical systems were also generally limited to working with constant, non-adjustable dwell time and limited authority over the spark time.
With the advent of the single chip microprocessor, electronic control systems replaced mechanical systems as the system of choice for controlling the ignition of high performance reciprocating engines. Unlike a mechanical ignition control system which provides a constant non-adjustable dwell time and spark time, an electronic ignition control system may adjust both the dwell and spark times as a function of the engine operating conditions and ambient conditions to achieve improved engine performance and efficiency. As an example, the electronic control may either advance or retard the spark time from its nominal spark time as a function of varying engine operating parameters and ambient conditions (e.g. engine speed, ambient pressure and temperature, etc. . . . ).
Any new aircraft system that relates to engine control and flight safety must be certified by the Federal Aviation Administration (FAA) to ensure the system is reliable. In an aircraft reciprocating engine, an event of concern to the FAA is the likelihood of sustained pre-ignition which occurs when the fuel/air mixture within the cylinder is ignited prematurely. Conceivably, pre-ignition could occur as a result of a failure in the ignition control system (e.g., a single point failure in the microprocessor) which leads to an improper series of spark commands to the spark plugs. Sustained pre-ignition may lead to increased temperature within an engine cylinder, which through a series of subsequent events could lead to an aircraft engine fire. To ensure such an event does not happen, the ignition control system must have sufficient reliability to ensure the probability of pre-ignition is less than one in a billion.